The field of the invention is heating and cooling, and the invention relates more particularly to air treating units such as furnaces, air conditions, humidifiers, and the like which include an air filter which tends to become clogged during use.
Many units have been devised for measuring the slight pressure difference across an air filter to show when the air filter has become clogged. Most of these units utilize a diagram which has the inherent disadvantage of becoming torn as well as providing a relatively expensive assembly. Diaphragm containing units are shown in U.S. Pat. Nos. 3,027,865; 3,916,817, and 3,934,543. A unit utilizing a conical sleeve around the which the air flows, is shown in the U.S. Pat. No. 3,587,515 and a unit which has a switch which is physically closed by the inward bowing of a clogged air filter, is shown in U.S. Pat. No. 3,928,006. All of these units are relatively expensive or are difficult to adjust. All of these units fail to show progressively the filter condition when the fan is off.